EFFECTS ON THE DISTANCE LAPLACIAN SPECTRUM OF GRAPHS WITH CLUSTERS BY ADDING EDGES

All graphs considered are simple and undirected. A cluster in a graph is a pair of vertex subsets (C,S), where C is a maximal set of cardinality vertical bar C vertical bar >= 2 of independent vertices sharing the same set S of vertical bar S vertical bar neighbors. Let G be a connected graph on n vertices with a cluster (C,S) and H be a graph of order vertical bar C vertical bar. Let G(H) be the connected graph obtained from G and H when the edges of H are added to the edges of G by identifying the vertices of H with the vertices in C. It is proved that G and G(H) have in common n - vertical bar C vertical bar +1 distance Laplacian eigenvalues, and the matrix having these common eigenvalues is given, if H is the complete graph on vertical bar C vertical bar vertices then partial derivative - vertical bar C vertical bar + 2 is a distance Laplacian eigenvalue of G(H) with multiplicity vertical bar C vertical bar - 1 where partial derivative is the transmission in G of the vertices in C. Furthermore, it is shown that if G is a graph of diameter at least 3, then the distance Laplacian spectral radii of G and G(H) are equal, and if G is a graph of diameter 2, then conditions for the equality of these spectral radii are established. Finally, the results are extended to graphs with two or more disjoint clusters.